This invention relates to a magnetic powder for magnetic recording media which can maintain proper or suitable coercive force (Hc) and also has a high saturation magnetization (.sigma.s) and to a magnetic recording medium whose electromagnetic transducing characteristics, especially reproduction output, may be improved by use of said magnetic powder.
There have been used magnetic recording media for video recording, digital recording, etc. prepared by coating the surface of a substrate with needle-shaped magnetic particles of such as .gamma.-Fe.sub.2 O.sub.3 and CrO.sub.2 and orientating the particles. In such cases, the particle size of the magnetic powder is required to be sufficiently smaller than the minimum recording unit to obtain a satisfactory S/N ratio. For example, in the case of video recording currently practiced, a needle-shaped magnetic powder having the length of about 0.3 .mu.m relative to the minimum recording wave length of about 1 .mu.m is employed. And recently a further improved recording density is desired. Accordingly, there is a strong demand for using a much finer magnetic particle rather than the needle-shaped magnetic powders currently used.
Now, as a magnetic powder for magnetic recording, the one with an easy axis of magnetization of uniaxial type is preferred. In other words, in the recording media currently used, the magnetic recording layer is furnished with uniaxial anisotropy, and signals are recorded therein in the direction of the easy axis of magnetization.
The present inventors have developed, as such a fine magnetic particle, a hexagonal crystalline system ferrite powder with a mean particle size of 0.3 .mu.m or less as disclosed in, for example, Japanese Patent Provisional Publication No. 86103/1980. Since said magnetic powder is not only of a finer particle compared with the conventional one formed by using .gamma.-Fe.sub.2 O.sub.3 or CrO.sub.2 but also a hexagonal platelet in its shape and has an uniaxial easy axis of magnetization perpendicular to the surface of the platelet. Accordingly, a coated type recording medium of the perpendicular magnetization system may possibly be prepared. In view of the facts mentioned above, it can be said that the hexagonal crystalline system ferrite powder is a magnetic powder suitable for high-density recording.
Since such a hexagonal crystalline system ferrite itself usually has a coercive force (Hc) too high to carry out magnetic recording, it is necessary to reduce the Hc by substituting a part of constituting atoms by some other specific atoms.
However, there occurs an inconvenience that the saturation magnetization (.sigma.s) of a substituted type hexagonal crystalline system ferrite which has been obtained through the substituting procedure mentioned above may decline compared with the same before substitution. For example, while in a case of Co-Ti substituted type Ba ferrite wherein the decline of s value is relatively small, the s value of Ba ferrite before substitution is approximately 70 emu/g; when the Hc of said ferrite is controlled down to 1,000 Oe or less by Co-Ti substitution, the .sigma.s value thereof may often decline to 60 emu/g or less. When compared with a conventional fine metal particle such as Fe metal or Fe-Co, Fe-Ni, Fe-Co-Ni alloy, etc., and further when compared with the conventional needle-shaped .gamma.-Fe.sub.2 O.sub.3 particle whose .sigma.s value is approximately 70 emu/g, the .sigma.s value of said ferrite may become lower.
The .sigma.s value of this magnetic powder is an important parameter by which the properties of a magnetic recording medium prepared by use of said magnetic powder, particularly on reproduction output, may be greatly influenced. And the decline of the .sigma.s induces the decline of reproduction output of the magnetic recording medium.
Thus, as a hexagonal crystalline system ferrite, the one whose Hc is adjusted at a proper or suitable value and also with high .sigma.s value is now strongly desired.